...if you're traveling internationally.
It's convenient, but it almost guarantees you'll get appointments screwed up if you're moving countries regularly.
There's so many quirks to timezones - a particular city, state, or province will often operate slightly differently than the ones around it. For instance, in the USA, Arizona doesn't do daylight savings time. So it's an hour off from the rest of its timezone half the year.
I find the best way to handle appointments when traveling around a lot is to mark down when they're going to happen in the timezone of the person I'm talking to or meeting. Then, I don't convert until the week the appointment is happening.
This avoids the problem of trying to remember when you marked China time or Japan time for an appointment if you're traveling between the two countries.